This invention relates to a voltage regulator with dynamically boosted bias current.
Conventional voltage regulators aim to maintain a constant voltage on a load through variations in load current and changes in supply voltage. Typically, an off-chip external pass device provides current to a load. The load voltage is maintained by an on-chip error circuit, which typically senses a fraction of the load voltage through a voltage divider and compares it to a reference voltage. A difference between the sensed load voltage and the reference voltage produces an error signal which is buffered by an on chip driver circuit to drive the pass device to reduce the error signal. Such driver circuits require a large static bias current which must always be present in order to provide adequate slew current in order to keep transient voltage changes and recovery times due to line or load changes within acceptable limits. The large static bias currents are a constant drain on the battery or other power supply sources. Attempts to decrease the static bias currents not only slow the transient response but increase the output impedance of the driver circuit and decrease bandwidth.
It is therefore an object of this invention to provide an improved voltage regulator with dynamically boosted bias current.
It is a further object of this invention to provide such an improved voltage regulator with dynamically boosted bias current which reduces changes in output voltage and output voltage recovery time in response to a change in load current or change in supply voltage.
It is a further object of this invention to provide such an improved voltage regulator with dynamically boosted bias current which achieves those goals with less static bias current.
It is a further object of this invention to provide such an improved voltage regulator with dynamically boosted bias current which achieves those goals while reducing the output impedance of the driver circuit.
It is a further object of this invention to provide such an improved voltage regulator with dynamically boosted bias current which extends the bandwidth of the driver circuit.
The invention results from the realization that a voltage regulator which reduces transient voltage changes and recovery time and reduces the output impedance of the driver circuit while extending the bandwidth of the driver circuit can be achieved with lower static bias current by sensing the error signal, comparing it to a reference signal and obtaining a second error signal which is gained up to boost the bias current of the driver circuit that controls the external pass device that provides current to a load.
This invention features a voltage regulator with dynamically boosted bias current including a pass device for providing current to a load. There is an error circuit responsive to a difference between a predetermined reference voltage and a function of the voltage on the load to produce an error signal. A driver circuit responsive to the error signal controls the pass device to adjust the current to the load to reduce the error signal. The driver circuit includes: an amplifier responsive to the error signal for controlling the pass device, a bias current source for biasing the amplifier, a sensing circuit for sensing a portion of the error signal, a reference current source for providing a reference current and a second error circuit responsive to a difference between the portion of the error signal and the reference current to produce a second error signal. A boost circuit responsive to the second error signal increases the bias current provided to the amplifier when the load demands more current.
In an preferred embodiment the pass device includes a transistor. The error circuit may include an error amplifier and a voltage divider for providing a fraction of the sensed voltage on the load to the error amplifier. The second error circuit may include a first current mirror responsive to the portion of the error signal and the reference current to produce the second error signal. The boost circuit may include a second mirror circuit for gaining up the second error signal to increase the bias current provided to the amplifier when the load demands more current. The portion of the error signal may be approximately an order of magnitude less than the error signal. The boost circuit may have a gain of at least one order of magnitude. The second error circuit may have a gain of approximately unity. The error circuit and driver circuit may be on chip and the pass device may be off chip.